The use of automation in determining nitrogen by the Kjeldahl method, with final calculations by computer

Abstract

An account is given of the development of a system making maximum use of automated equipment in the routine determination of nitrogen in biological samples when the desired order of precision is better than 1 per cent. and the expected number of analyses is greater than 100 per day.

An automatic digestion unit made by Technicon Instruments Company Ltd. tested for liquids did not achieve the accuracy desired. Results for biological fluids with this digestor tended to be low on occasion by up to 5 per cent. Recovery of nitrogen from solutions of the pure compounds creatine and arginine was as low as 70 per cent. and alterations in the digestion mixture and digestor heating conditions did not materially improve recovery. For this reason all samples were subsequently digested by a standard Kjeldahl method before automated determination of ammonia in the diluted digests.

Studies were made of the indophenol-blue reaction used in the automated system for determining ammonia in aqueous solution. Conditions were most stable when the reaction coil was maintained at 22°± 1° C and dwell time in the coil was about 3 minutes. Effects of mineral content of test solutions, sampling rate and sampler cam irregularities on the reliability of results were investigated.

Results for nitrogen determined in acid digests by the automated procedure and by a standard distillation method showed close agreement.

The use of an I.B.M. 1130 computer allowed calculation of results to be carried out in half the time taken when using an electronic desk calculator.